In an optical transmission system, to transmit an orderwire signal or a supervisory and control signal, it is advantageous, in view of effective utilization of an optical fiber, to superimpose these subsignals on a main signal passing through the optical fiber.
These subsignals are usually transmitted to or received from a line terminating equipment.
In recent years, however, accompanied by the developments in large capacity optical transmission systems, requirements have increased to drop or insert the subsignals at any desired repeater such as in a regenerator or, in other words, an intermediate repeater arranged between line terminal equipments. Such an intermediate repeater having a drop/insert function is referred to in this specification as a D/I repeater. When a D/I repeater is provided, then, between these D/I repeaters, or between the line terminal equipment and the D/I repeater, subsignals such as the orderwire signal or the supervisory and control signal can be transmitted and received through two or more specified channels. These subsignals are required to be dropped or inserted at a desired D/I repeater.
In the conventional intermediate repeaters, drop/insert units for dropping or inserting necessary signals are provided for all channels regardless of whether or not the signals on the channels need not be dropped or inserted. In the intermediate repeaters, however, there are signals such as communication data which need not be dropped or inserted from or into channels. Hereinafter these signals which need not be dropped or inserted are referred to as through channel signals. Therefore, there is a problem in that the number of the drop/insert units (hereinafter also referred to as D/I units) in a conventional intermediate repeater is so large that the circuit scale of the conventional intermediate repeater is large.
In particular, for high-speed data, since it is difficult to directly drop or insert subsignals from or into high-speed data, the high-speed data is usually converted into a plurality of low-speed data channels of for example 400 Mb/s by a serial-parallel conversion, and the subsignals are dropped or inserted from or into one of the parallel channels. In this case, a suitable channel selecting circuit has to be developed.
If a limited number of drop/insert units are provided to correspond to only the necessary channels other than the through-channels, the circuit for selecting the through-channels must be as simple as possible. However, conventional circuits for selecting these through-channels are very complex. Namely, these circuits are formed by a random logic circuit constructed of 20 to 30 integrated circuits (IC's) so that the circuit scale becomes large and is opposite to the requirement to construct a D/I repeater as simple as possible.
From another point of view, if the intermediate repeater has the drop/insert function, the constitution thereof becomes similar to that of the line terminal equipment so that these repeaters can be manufactured with the same design. Conventionally, however, the line terminal equipment and the intermediate repeater are manufactured with different system designs. The different system designs cause disadvantages in high manufacturing cost, in complex control of the repeaters, and in preparing different kinds of spare repeaters for accidental faults.